1. Field of the Invention
The present invention relates to a method of manufacturing a porous metal sheet which is preferably used as an electrode substrate of a battery; the porous metal sheet manufactured by the manufacturing method; and an electrode for a battery using the porous metal sheet. More particularly, the present invention relates to a porous sheet formed of metal powders so that an active substance is filled into pores thereof. The porous metal sheet is preferably used as the electrode substrate of a nickel hydrogen battery, a nickel cadmium battery, a lithium primary battery, a lithium secondary battery, an alkali dry cell, a fuel cell and soon; and an electrode plate of various batteries, for example a battery for vehicles.
2. Description of the Related Art
As porous metal sheets of this type which are used as the electrode substrate of batteries, the present applicant proposed various kinds of metal porous materials made of a foamed material, a nonwoven sheet, a mesh material, a laminated sheet comprising two or more thereof by treating these materials so that they are electrically conductive and then electroplating them.
In manufacturing the porous metal sheet by the method, it is necessary to perform an electric conductive treatment by a method such as vaporizing method, chemical plating method or carbon application method as a pre-treatment of electric plating. It is troublesome and costly to carry out these methods. Further, when the foamed material, the nonwoven sheet, the mesh material are electroplated and burnt to remove resinous material and metal powders are sintered, burnt-off portions are cavitied. Thus, an active substance cannot be filled thereinto.
In view of the problems, the present applicant proposed many methods of manufacturing the porous metal sheet from metal powders.
In any of the above-described methods, using adhesive agent, fine metal powders are applied to entire surfaces including the inner surfaces of pores of the foamed material, the nonwoven sheet, the mesh material or the laminated sheets thereof so as to form a conductive metal layer thereon. Then, the resinous material is removed and the metal powders are sintered to form porous metal sheets.
In the above-described conventional methods of forming the porous metal sheet, using fine metal powders, fine metal powders are applied to the surface of the porous base plate such as the foamed material, the nonwoven sheet, the mesh material, or the laminated sheets thereof. Thus, the size and shape of the pore of the porous metal sheet is restricted by the size and shape of the pore of the porous base plate. Thus it is difficult to form a pore smaller or greater than the pore of the porous base plate and a pore whose shape is different from that of the pore of the porous base plate.
As one of the conditions required as the substrate of the electrode plate, it is necessary that the porous metal sheet is thin to accommodate a large amount of an active substance so as to improve the performance of a battery. But in the above-described conventional methods of forming the porous metal sheet, using fine metal powders, the thickness of a porous metal sheet is restricted by the thickness of the base plate. Hence it is difficult to manufacture a porous metal sheet having a thickness 1 mm or less.
Further, adhesive agent is used in any of the above-described conventional methods. That is, fine metal powders are applied to the base plate by mixing them with the adhesive agent or after the adhesive agent is applied to the base plate. Therefore, the adhesive agent is present between adjacent fine metal powders. Thus when the adhesive agent is burnt off, together with the base plate during the removal of the resinous material and the sintering of the fine metal powders, large gaps are formed between the adjacent fine metal powders. As such, it is difficult to control the shape and size of the pore. In addition, the number of processes is increased because the adhesive agent is used.
Further, a solid metal foil is hitherto used as the substrate of the positive and negative electrodes of a lithium secondary battery. In this case, lithium ions are incapable of moving from the front surface of the electrode substrate to the rear surface thereof and from the rear surface thereof to the front surface. Therefore, in order to obtain a possible most uniform and thinnest active substance layer, the active substance is required to be applied to one surface of each of the positive and negative electrodes. In addition, because the surface of the substrate is smooth, the active substance is liable to be separated from the base plate.
In porous metal sheet having punching shape, lath-shape, mesh-shape, foamed sheet-shape, nonwoven sheet-shape and so on, lithium ions are capable of moving from the front surface of the electrode substrate to the rear surface thereof and vice versa and further, the thickness of the active substance can be controlled at both the front and rear surfaces of the electrode substrate by the porous metal sheets. Thus, research and development are being made to use such the porous metal sheets as the electrode substrate of the lithium secondary battery. But the size of the solid portion of the conventional porous metal sheet and the size of the pore thereof are not uniform. Therefore, the lithium ions are incapable of moving uniformly and sufficiently. Although it is preferable that the porous metal sheet has a large number of small pores thereon to allow the lithium ions to move smoothly, the porous metal sheet which satisfy such a demand has not been proposed.
The electrode substrate of the lithium secondary battery is required to have a thickness of 10 xcexcm-30 xcexcm. But as described above, it is difficult for the conventional art to manufacture a porous metal sheet having a thickness 1 mm or less. That is, it is impossible to manufacture a porous metal sheet whose thickness is as small as a thin foil.
In recent years, portable equipment such as a video camera, a liquid crystal compact television, a CD player, and the like requiring high current have come into wide use. Thus, there is a growing demand for the development of batteries having a great discharge capacity and superior in discharge characteristic in a high load-applied state. But in the conventional alkali dry cell having a separator, pellets having positive electrode is filled outside the separator and gel powdered zinc is filled inside the separator. As such, it is very difficult for the alkali dry cell to have a great discharge capacity and have an improved discharge characteristic in a high load-applied state because the battery can has a limited space.
In order to solve the problem, researches are made on an alkali dry cell whose negative electrode plate consists of a punching or expanding solid zinc foil or zinc foil and positive electrode plate consists of metal oxide. The negative electrode plate and the positive electrode plate are spirally coiled with a separator interposed to increase the area of the negative electrode plate and the positive electrode plate and the discharge capacity of the battery and improve the discharge performance thereof in a high load-applied state. But the punching or expanding zinc foil has a problem that the open area ratio is about 50% or less because pores are two-dimensional; pore-forming processing is carried out and thus a pore-formed portion is cut off, and hence material left parts of material are much; processing and material costs become high as the thickness of the zinc foil is increasingly thin; and strain and burr are liable to appear in the pore-forming process. Further, the solid zinc foil and the conventional porous metal sheet have problems similar to those of the lithium secondary battery.
Further, conventionally, the electrode of alkali secondary batteries such as the nickel hydrogen battery and the nickel cadmium battery is formed as follows; paste-like slurry of an active substance formed by mixing an active substance such as hydrogen-storing alloy powder or powder of nickel hydroxide and electrically conductive agent comprising carbon, binder and so on is applied to a collector such as punching metal, metal mesh, expanded metal. But the binder prevents flow of electric current, thus making the electricity-collecting property in the thickness direction of the electrode worse.
The present invention has been developed to solve the problems and to improve a manufacturing method of a porous metal sheet made from metal powders. It is an object of the present invention to provide a porous metal sheet-manufacturing method capable of controlling the thickness thereof, the size of pores, the shape thereof as desired and eliminating the use of adhesive agent so as to manufacture the porous metal sheet in a simple process; and the porous metal sheet which is manufactured by the method.
It is another object of the present invention to provide an electrode which comprises a metal sheet into which a binder-unadded active substance consisting of powders can be charged and is superior in electricity-collecting performance.
In order to solve the problems, there is provided a method of manufacturing a porous metal sheet comprising the steps of:
spreading metal powders on a feeding belt which is continuously fed;
passing the feeding belt on which the metal powders have been spread through a sintering oven; and
sintering the metal powders, with adjacent uncompressed metal powders in contact with each other partly and gaps present therebetween so as to integrate contact portions of the metal powders with each other and form the gaps as fine pores.
The feeding belt comprises a solid metal sheet, an inorganic sheet including a porous metal sheet; or a laminated sheet of the solid metal sheet and the inorganic sheet including a porous metal sheet of a circulating driving device of belt conveyor type. For example, the feeding belt is SUS (310S). Metal powders spread on the feeding belt can be separated therefrom by sintering them to form into a sheet. That is, the metal powders can be formed into a porous metal sheet continuously with high efficiency by passing the feeding belt being continuously fed through the sintering oven.
As described above, because the metal powders spread on the feeding belt are kept uncompressed, spherical surfaces of adjacent metal powders are not in contact with each other but in a dot contact state or in a line contact state, and thus gaps are present between the adjacent metal powders. When they are heated in this state at a required temperature in a sintering oven, contact portions of the adjacent metal powders are integrated with each other. As a result, the gaps between the metal powders are formed as fine pores and thus a porous metal sheet can be continuously formed.
Accordingly, the size of a pore depends on the size of a metal powder. That is, a large pore is formed when the diameter of the metal powder is large, whereas a small pore is formed when the diameter of the metal powder is small. The metal powder having a diameter in a range of 0.1 xcexcm-100 xcexcm is preferably used.
Metals which are used as the material of the porous metal sheet are not limited to specific kinds. The following substances are preferably used: Ni, Cu, Al, Ag, Fe, Zn, In, Ti, Pb, V, Cr, Co, Sn, Au, Sb, C, Ca, Mo, P, W, Rh, Mn, B, Si, Ge, Se, La, Ga and Ir. Each metal described above is used in the form of oxide and sulfide thereof and a simple substance or a mixture, including compounds of these metals. That is, Al, Ti and V which cannot be used in electroplating can be used. It is possible to use one kind of metal selected from these metals in the form of powders or a mixture of a plurality of these metals in the form of metal powders. It is desirable that metal powders of these metals do not intertwine with one another and are dispersed favorably. That is, the peripheral surfaces of the metal powders are desired not to be convex or concave so that they do not intertwine one another. Thus, it is preferable that the metal powders are spherical, dice-shaped, square pillar and columnar.
Because the feeding belt is purous, metal powders spread thereon drop from the pores of the feeding belt. Consequently, through-holes are formed on the resulting porous metal sheet. The through-holes are larger than pores consisting of small gaps present between the metal powders. As such, the resulting porous metal sheet has the large through-holes and the fine pores. Metal powders which have dropped from the pores of the feeding belt are collected to recycle them.
Further, there is provided a method of manufacturing a porous metal sheet comprising the steps of feeding a supporting sheet continuously; spreading metal powders on the supporting sheet; feeding the supporting sheet on which the metal powders have been spread on a feeding belt; and passing the supporting sheet through a sintering oven, together with the feeding belt; and sintering the metal powders on the supporting sheet, with adjacent uncompressed metal powders in contact with each other partly and gaps present therebetween so as to integrate contact portions of the metal powders with each other and form the gaps as fine pores.
The supporting sheet comprises an organic sheet including a solid resinous sheet, a three-dimensional reticulate resinous sheet and a porous fibrous resinous sheet, an inorganic sheet including a solid metal sheet and a porous metal sheet or a laminated sheet composed of a plurality of sheets selected from the sheets.
Because the supporting sheet is used, the resulting porous metal sheet can be separated from the feeding belt more easily than the porous metal sheet formed on the feeding belt by directly spreading metal powders thereon. A resinous sheet used as the supporting sheet is burnt off in a resinous material-removing oven. An inorganic sheet such as a metal sheet is not removed by heating. Some inorganic sheets are separated from the resulting porous metal sheet, whereas some inorganic sheets are not separated therefrom but fed downstream, together with the feeding belt and wound around a roll. The supporting sheet formed of a thin metal plate can be fed at a high speed and thus a high productivity can be obtained.
When a sheet having a large number of holes is used as the supporting sheet, as same as the case of the feeding belt, it is possible to manufacture a porous metal sheet having fine pores surrounded with the metal powders and large through-holes formed in portions corresponding to the holes of the sheet used as the supporting sheet.
The feeding belt or the supporting sheet on which the metal powders have been spread is passed through a cooling oven positioned subsequently to the sintering oven to cool the metal powders after the metal powders are sintered.
Preferably, after the metal powders are sintered and cooled, the resulting porous metal sheet is passed between a pair of rolling rollers to increase an area of integrated portions of the metal powders with each other so as to increase the strength of the porous metal sheet.
Preferably, the sintering, the cooling, and the rolling are repeated at a plurality of times.
Further after the metal powders are sintered, cooled, and rolled, the resulting porous metal sheet is preferably separated from the feeding belt or the supporting sheet.
That is, it is possible to use the porous metal sheet formed by sintering metal powders as an electrode substrate. When the porous metal sheet does not have a desired strength because pores thereof are large and the area of integrated portions of the metal powders is small, the area of integrated portions thereof can be increased by rolling the obtained porous metal sheet lightly. If a great force is applied to the porous metal sheet at a time, it may be meandered or cracked. Therefore, it is preferable that the porous metal sheet is rolled at a small rolling coefficient at a plurality of times.
Preferably, metal powders are spread again on a surface of the porous metal sheet formed by the sintering and sintered so that the thickness of the porous metal sheet is increased to a required one and the tensile strength thereof is improved.
Further, preferably, after the metal powders are spread on the feeding belt or the supporting sheet, the metal powders are pressed at a required small pressure by a pressing roller, and then, the metal powders are passed through the sintering oven to sinter the metal powders.
The pressing roller is used not to compress the metal powders but increase the contact area thereof. The metal powders can be integrated with one another in a great area and the strength of the porous metal sheet can be enhanced by sintering it after pressing it lightly by the pressing roller.
Furthermore, the supporting sheet is preferably burnt off in a resinous material-removing oven.
The supporting sheet may not be burnt off in the resinous material-removing oven but the supporting sheet and the porous metal sheet formed of the metal powders may be laminated one on the other. That is, a porous metal sheet having a laminated structure. Using various kinds of supporting sheets selectively, it is possible to manufacture porous metal sheets of various forms by forming the laminated structure.
Metal sheets listed below may be used as the supporting sheet to laminate the porous metal sheet formed of metal powders one on the other to form the laminated structure. Further, after forming a porous metal sheet of metal powders, the porous metal sheet and the metal sheets list below are laminated one on the other and integrated with each other to manufacture a porous metal sheet having the laminated structure.
The metal sheets include a solid metal plate or a solid metal foil; a metal plate or foil having a large number of pores formed thereon; a metal mesh; a metal screen or/and three-dimensional reticulate foamed sheet; a porous fibrous resinous sheet; a mesh sheet or a porous metal sheet comprising a laminated sheet of these sheets; the porous metal sheet formed by removing a resinous material and sintering after plating the laminated sheet with a metal, evaporating a metal thereon, applying fine metal powders or spraying a metal thereto; a porous metal sheet formed of metal fibers; a porous metal sheet made of metal powders rolled by a pair of rolling rollers, at least one of which serves as a pattern roller; and a laminated sheet comprising these sheets laminated one on the other and integrated with each other.
It is possible to laminate the porous metal sheet manufactured by the method of the present invention; a metal plate or foil having a large number of pores formed thereon; a metal mesh; a metal screen; a three-dimensional reticulate foamed sheet or a nonwoven sheet-shaped porous metal sheet on both surfaces of the porous metal sheet manufactured by the method of the present invention by differentiating the size of pores of the porous metal sheet positioned on one surface of the laminated sheet and that of pores of the porous metal sheet positioned on the other surface thereof and differentiating the open area ratio or/and the diameter of the porous metal sheet.
A sheet having convexes and concaves formed alternately with each other is used as the feeding belt or the supporting sheet, so that metal powders which have been spread on the convexes are dropped to the concaves by vibrating the metal powders or by a scraping means and then sintered in a sintering oven so as to form fine pores between the adjacent metal powders and pores consisting of large through-holes corresponding to the convexes.
Because concaves and convexes are formed on the feeding belt and the supporting sheet on which metal powders are spread, the metal powders drop from the convexes to the concaves and accumulate in the concaves. Metal powders accumulated on the surfaces of the convexes are dropped to the concaves by vibrating the feeding belt or the supporting sheet. When the metal powders dropped to the concaves are sintered, a porous metal sheet is obtained, with the metal powders integrated with each other partly and pores present among the metal powders. Therefore, as in the case of the supporting sheet having holes formed thereon, the porous metal sheet thus manufactured has fine pores and comparatively large through-holes.
Pores of a porous sheet which is used as the feeding belt and the supporting sheet and the convexes are random shapes such as circular, rhombic, polygonal or elliptic. The pores and the convexes are preferably formed lengthwise and widthwise at predetermined intervals.
Because the pores or the convexes are formed on the feeding belt and the supporting sheet, a porous metal sheet having through-holes corresponding to the pores or the convexes is formed. If the pores or the convexes are circular, the formed porous metal sheet has hole-shaped pores. If the pores or the convexes are rhombic, pores of the formed porous metal sheet are lath-shaped.
A sublimable fine fragment which is burnt off by heating is mixed with the metal powders or spread on the feeding belt or the supporting sheet before the metal powders are spread thereon, and the sublimable fine fragment are burnt off in a resinous material-removing oven to form fine pores between the adjacent metal powders and large pores formed in portions where the sublimable fine fragment has been present.
When some agent such as foaming agent generating gas when it is heated is used as the sublimable fine fragment, a resulting porous metal sheet has through-holes formed by the generated gas. Further, the size of the through-holes can be controlled according to the diameter of particles of the sublimable fine fragment.
Further, in the present invention there is provided a porous metal sheet manufactured by the method in above-mentioned.
The porous metal sheet formed by using the sublimable fine fragment or/and the feeding belt and the supporting sheet-consisting of a porous sheet or a material having convexes and concaves formed alternately with each other, is punching shape, reticulate shape, honeycomb-shape, lath-shape, lattice-shape, expanded sheet-shape, screen-shape or lace-shape. That is, a porous metal sheet having a desired shape can be manufactured according to the shape of the sublimable fine fragment, the shape of the pore of a porous sheet, the shape of the convex.
The porous metal sheet has preferably pore-unformed lead portions spaced at predetermined intervals.
Further, in the present invention, there is provided a substrate for a battery electrode comprising the porous metal sheet formed by the above method.
Further, in the present invention, there is provided an electrode for a battery in which an active substance is charged into a pore of the substrate for a battery electrode, and an active substance layer is formed on at least one surface of the substrate for a battery electrode.
As the active substance, the following substances can be used: metals such as zinc, lead, iron, cadmium, aluminum, lithium, and the like; metal hydroxides such as nickel hydroxide, zinc hydroxide, aluminum hydroxide, iron hydroxide, and the like; complex oxides such as lithium dimanganese tetraoxide, lithium cobalt dioxide, lithium nickel dioxide, lithium divanadium tetraoxide and the like; metal oxides such as manganese dioxide, lead dioxide, and the like; electrically conductive polymers such as polyaniline, polyacethylene, and the like; hydrogen-storing alloy; carbon; and other substances. The kind is not limited.
Conventionally, when the active substance is charged into a substrate for a battery electrode, an electrically conductive material such as carbon powders and binder are added to the active substance. But according to the present invention, the active substance is used without adding the binder thereto. The porous metal sheet of the present invention has fine pores into which powders of the active substance can be charged without binding them by the binder. In particular, when a pore has a three-dimensional structure, powders of the active substance can be held at a high strength and thus can be reliably held without being dropped from the porous metal sheet. The electricity-collecting performance of the electrode can be outstandingly enhanced by not adding the binder to the active substance.
In the case of the negative electrode of a nickel hydrogen battery, powders containing hydrogen-storing alloy as the main component is used as the active substance. The active substance consists of hydrogen-storing alloy powder or mixture of the hydrogen-storing alloy powder and a transition metal. Further, it is preferable that a surface of the active substance layer is covered partly or entirely with the transition metal.
It is preferable that an active substance containing hydrogen-storing alloy as a main component is successively supplied at a required pressure to a porous metal sheet successively formed by the method at above-mentioned to fill pores of the porous metal sheet with the active substance, and an active substance layer having a required thickness is formed on at least one surface of the porous metal sheet. That is, the electrode can be successively manufactured by successively supplying powders of the active substance to the porous metal sheet at a required pressure after the process of forming metal powders into the porous metal sheet which constitutes an electrode substrate finishes.
Further, in the present invention, there is provided a battery comprising the electrode for a battery. As the battery, a nickel hydrogen battery, a nickel cadmium battery, a lithium primary battery, a lithium secondary battery, an alkali dry cell, a fuel cell; and a battery for vehicles are exemplified.